1. Field of the Invention
The present invention relates to vacuum type circuit interrupters and more particularly to a contact structure used in such vacuum interrupters.
2. Description of the Prior Art
In a vacuum interrupter the current normally flows through a pair of contacts located in an evacuated envelope. These contacts are relatively movable between a closed position in engagement and an open position spaced apart to interrupt the current flow through the vacuum interrupter. As the contacts are separated an arc is formed therebetween and current will continue to flow until this arc is extinguished, which on an alternating current circuit will normally occur near the first current zero. The contact surface must support the arc from its initiation at the time of contact separation until its extinction at approximately current zero. While the arc is being sustained the contacts are subjected to very intense localized heating. The dissipated arc energy causes melting, erosion and general deterioration of the contact surfaces. To minimize deterioration of the contact surfaces, it is customary to move the arc around the contact surface. Such arc movement tends to minimize the amount of metallic vapors or particles generated by the arc from the contact surfaces during circuit interruption. This arc movement is usually accomplished by self-induced magnetic fields as more fully explained in prior art U.S. Pat. No. 3,089,936 issued May 14, 1963 to S. R. Smith, Jr. and U.S. Pat. No. 3,417,216 issued Dec. 17, 1968 to S. R. Smith, Jr.
French Pat. No. 1,410,884 shows a cup-shaped contact with an inward facing lip. This contact however, does not have a large continuous contact making surface, as disclosed in the instant application, and thus would have the problems as discussed for U.S. Pat. No. 3,836,740. British Pat. No. 997,384 discloses a cup-shaped contact having slotted side walls wherein the slots do not extend to the contact surface. This contact however, does not have the inward facing lip portion, which is necessary for large current interrupting ability. Also British Pat. No. 997,384 does not teach that the contact surface and the main contact body can be made of different material which in some applications is very important.
In U.S. Pat. No. 3,836,740 a contact construction is disclosed wherein a cup-shaped contact with an inwardly facing lip is provided. The lip portion and the side walls are slotted, thus moving an arc formed during circuit interruption radially inward and circumferentially around the contact surface formed by the lip portion. Specific advantages and construction of this contact are more fully disclosed in copending U.S. Pat. No. 3,836,740. An unobvious problem with this construction is that a large amount of arc-seasoning is required to bring the interrupting ability of this contact to its full potential. This large amount of preconditioning restricts the use of this contact construction in commercial apparatus.